Polyurethane foams are widely used as materials from which articles such as mattresses, seat cushions, and thermal insulators are fabricated. Such polymeric foam materials are ordinarily manufactured by a casting process in which a mixture of liquid polyurethane-foam-generating reactants are deposited in a mold. As used herein, the term "mold" includes both stationary molds for batch casting and translating or otherwise movable molds for continuous casting. Evolution of a gas causes the reactants to foam. For some foam formulations, the reactants themselves react to evolve sufficient gas; in others a blowing agent is mixed with the reactants to provide gas evolution. Continued gas evolution causes the foam to expand to fill the mold. The foam, initially a partially expanded fluid mixture, becomes increasingly viscous as the reactants polymerize, ultimately curing into a polyurethane foam casting shaped by the mold.
Slabs of polyurethane foam approximately rectangular or circular in cross section are conventionally cast in a translating channel-shaped mold. Such molds typically include a belt conveyor forming the bottom of the mold and a pair of spaced-apart, opposing side walls, which can be fixed or translatable at the speed of the conveyor. The mold sides and bottom are generally lined with one or more sheets of flexible-web such as kraft paper or polyethylene film. The sheets of mold liner are ordinarily withdrawn from rolls and continuously translated along the mold channel at the same speed as the belt of the conveyor. Liquid-foam-generating reactants are deposited on the mold bottom in a zig-zag pattern from a pouring nozzle positioned above the mold which is reciprocated back and forth across the width of the mold. Typically, after the reactants flow together, they expand and form a uniform slab of foam. Conventional production of foam products having rectangular and circular cross sections is taught in U.S. Pat. No. 3,325,823 to Boon and U.S. Pat. No. 3,325,573 to Boon et al., respectively. The disclosures of those patents are incorporated by reference.
If fresh reactant mixture is deposited on top of foam generated from previously deposited reactants, the resulting cured foam will have an uneven surface and non-uniform density, which is undesirable for most applications. By continuously translating the mold liner, the reactant mixture is continuously carried away from the pouring area below the pouring nozzle, which reduces the tendency for fresh reactant mixture to cover that previously deposited.
Propitious selection of conveyor velocity can prevent production of undesirable foam products. A range of velocities can be established for a particular reactant mixture formulation. Minimum velocity is achieved when liquid reactant mixture is evenly distributed on the bottom of the mold and does not flow in a direction opposite to that of the mold and conveyor. Maximum velocity is achieved when the deposited liquid mixture begins to flow in the same direction as the conveyor.
Selection of a velocity within the mentioned range requires consideration of the chemical reaction occuring subsequent to the depositing of liquid mixture in the mold. During residence in the mold, liquid mixture foams and cures. The height of the foam is affected by conveyor velocity. Because economy necessitates maximum product height, lower velocities are preferred during the foaming portion of the reaction to attain such heights. The ratio of conveyor velocity to product height is a useful criterion for evaluating process efficiency, i.e. the lower the ratio, the more efficient the process. According to the process of this invention, that ratio can be reduced to the range of about 1 to about 3.
To reduce further the tendency of the liquid reactants to flow back under the pouring nozzle and to assist the "zig-zags" of reactant mixture to merge uniformly, it is customary to incline a pouring board, the surface under the nozzle, from horizontal so that the bottom liner slopes downward in the direction of translation. However, the angle of inclination of the pouring board cannot be greater than about 4.5.degree. from horizontal for typical flexible polyether polyurethane-foam formulations without causing the reactant mixture to flow forward under previously deposited mixture, which leads to undesirable nonuniform foam. The angle of inclination is different for different foam formulations, such as polyester polyurethane foams.
Problems arise if the mold bottom slopes downward along its entire length. Conventional continuous slab molds are quite long, typically in excess of 60 feet, to provide for the long curing time of the foam. Building a translatable mold of this length inclined from horizontal is significantly more expensive than building a translatable mold of the same length which is horizontal, because, for example, the building housing the inclined mold would be required to have higher than normal ceilings. Moreover, it is expensive to change the angle of inclination of the entire mold to compensate for differing viscosities among various foam formulations. Thus, some continuous slab molds have horizontal belt conveyors for most of the length of the mold bottom, but have relatively short inclined pouring boards located beneath the pouring nozzles. The expansion and rise of the foam generally takes place on the sloping pouring board.
A second reason for providing a pouring board which makes an angle with respect to the belt conveyor relates to the cross-sectional shape of the slab cast in the mold. As the foam expands and rises in the mold, it encounters the sides of the mold. If the mold-side liners are being translated substantially parallel to the mold bottom, the expanding foam experiences a shear force which resists its rise along the sides. This shear force results in a rounding of the top of the slap to form a crown or crest of convex shape, much like a loaf of bread. For most applications, such rounded portions are unusable and must be discarded as scrap. Thus, the more nearly rectangular the cross section of the slap, i.e., the flatter the top, the more economical is the casting process.
If, over the length the foam expands, the mold bottom liner and the two mold side liners are translated at an angle with respect to one another, the mold side liner can have a velocity component relative to the mold bottom in the direction of the expansion of the foam. This velocity component can compensate for the shear force which resists the rise of the foam. Guiding the mold bottom liner across an inclined pouring board, which intersects an inclined mold-bottom conveyor can provide such a compensating velocity component when foam expansion occurs over the length of the pouring board and when mold-side liners are translated parallel to the mold-bottom conveyor. The angle of intersection, which ordinarily leads to polyurethane foam slabs having the most nearly rectangular cross sections, is about 10.degree. for typical foam formulations and production conditions. Unfortunately, if the pouring board is sloped 10.degree. from horizontal freshly deposited reactant mixture tends to flow forward, as discussed above, leading to foam slabs of non-uniform density or other imperfections.
Although it is possible to construct a continuous slab mold with a pouring board inclined from horizontal by an angle of 4.5.degree. and intersecting the belt conveyor at 10.degree., the belt conveyor in such a case must be inclined upward by an angle of 5.5.degree.. See, for example, the apparatus of U.S. Pat. No. 3,325,823. As noted above, however, inclined translatable molds are more expensive than comparable horizontal molds.
U.S. Pat. No. 3,786,122 discloses a process for producing polyurethane foam slabs which employs a horizontal, channel-shaped mold having at its forward end an inclined "fall plate" which makes an angle of significantly greater than 4.5.degree. from horizontal. The problem of reactant mixture flowing down the inclined fall plate is obviated by prereacting the reactant mixture prior to introducing it onto the fall plate. The prereacting step is carried out in a trough which opens onto the upper edge of the fall plate. Liquid foam reactants are introduced onto the bottom of the trough and the foam which is generated is allowed to expand upwards in the trough and spill over onto the fall plate. The foam continues to expand as it is carried down along the fall plate by a translating bottom sheet. Because the prefoamed reactant mixture exiting the trough is more viscous than the initial liquid reactant mixture, the fall plate can be inclined at a greater angle from horizontal than a pouring board in a conventional polyurethane foam slab mold.
An additional result of introducing prefoamed reactant mixture into the mold is that relatively high foam slabs can be produced as compared with conventional processes. Economies result from producing high slabs because, the thicker the foam slab, the less is the loss from discarding rind which generally coats polyurethane foam castings. With a conventional slab mold, if the rate of introduction of reactant mixture is kept constant and the rate of translation of the mold liner is reduced, the height of the foam slab tends to increase because more foam-generating reactant is deposited per unit length. However, if the rate of translation is slowed sufficiently, the expanding foam, particularly the youngest and most fluid portion, becomes unstable and tends to slip and shift, which results in cracks and other imperfections in the cured foam.
This problem of instability of rising foam is reduced in the process of the U.S. Pat. No. 3,786,122 by introducing into the translating mold prefoamed reactant mixture which is sufficiently viscous as to be able to sustain a relatively steep slope of the pouring board as it completes its expansion. Thus, the height of the foam can be increased. In addition to permitting higher foam slabs to be cast by reducing the translation speed of the mold liner, this process permits the use of slab molds shorter than those of conventional processes, because the slab moves a shorter distance during the curing time.
Certain problems attend the use of the open trough of the U.S. Pat. No. 3,786,122. For example, changing the width of the trough is difficult because foam deposits interfere with re-establishing fluid-tight seals. Moreover, the trough opening is subject to partial blockage by deposits of cured foam along the back and sides where the flow of prefoamed reactant mixture stagnates. Such deposits break free from time to time and are swept over the weir into the rising foam, thereby causing objectionable non-uniformities in the foam slab. A further difficulty is encountered when air bubbles are introduced into the bottom of the trough with the liquid reactants. These air bubbles generally remain entrained in the foam, leading to voids and other defects in the cured material.
U.S. Pat. No. 3,870,441 discloses an apparatus for producing polyurethane foam slabs which uses a horizontal, channel-shaped mold similar to the apparatus disclosed in the U.S. Pat. No. 3,786,122. Likewise, liquid foam reactants are introduced into the bottom of the trough and allowed to expand upward eventually spilling over onto a fall plate. The expanding foam moves across the fall plate to a conveyor via a translating bottom sheet. The improvement of the U.S. Pat. No. 3,870,441 is directed to a means for assisting the expanded foam in spilling over the fall plate. That flow assisting means comprises translating sheets, substantially perpendicular to the bottom sheet, which continuously move around the periphery of the channel-shaped mold including the periphery of the trough. However, that improvement to the U.S. Pat. No. 3,786,122 does not alleviate all of the drawbacks noted above for the U.S. Pat. No. 3,786,122.